1. Technical Field
This invention relates generally to a cover member for sealing one side of a circuit board, and more particularly, to such a cover member that has a multi-layered laminate structure.
2. Background Art
Electrical instruments, and radio equipment in particular, used in outdoor environments are subjected to severe weather conditions and harsh operational demands. Typically, a cabinet or an enclosure of some type is used to enclose the radio equipment and provide a weather-resistant housing for the electronic components of the equipment. The interior environment of the cabinet is often controlled to a level suitable for the enclosed electronic equipment and components. For example, the inside environment may be air-conditioned, fan-cooled, or heated. Many environmental control systems are sealed and use closed-system air conditioners and heat exchangers to prevent outside air from entering the enclosure. In applications where the complexity and/or cost of the sealed systems is prohibitive, outside air is generally used directly to cool the equipment inside the enclosure. Unfortunately, atmospheric air often contains many substances that will attack radio equipment, such as water, nitrates, hydrocarbons, sulphur dioxide, nitrogen oxides, hydrogen sulphide, chlorine, ozone, and salt.
Several methods are used for protecting circuit boards from harsh ambient environments. Conformal coatings have been used successfully for many years. Conformal coatings cover an entire circuit board to seal and protect the components on the board as well as the board itself. In this method, the circuit board is dipped or sprayed with a specified coating. However, it has been found that this method sometimes leaves small holes or gaps in the coating, and can have an uneven buildup of material, creating problems for handling and circuit performance. More recently, vacuum deposition has been used to build up the protective layer at a molecular level. However, vacuum deposition is an expensive and time-consuming process, and requires a significant capital investment in specialized equipment.
Potting is another form of conformal coating wherein the circuit board is permanently fixed inside a small box or frame and potting compound poured over and around the board, imbedding it completely in the compound. However, potted circuit boards require increased physical space and make repair of the circuit board very difficult, if not impossible.
Placing the circuit board within an air-tight enclosure is also a method of sealing the circuit board. For example, U.S. Pat. No. 5,527,989, issued Jun. 18, 1996 to Karl-Erik Leeb, describes a device for encapsulating electronic components. However, although a circuit board can be encapsulated within the described device, all components that require visual or physical accessibility, such as connector assemblies, are not covered and hence not protected from airborne contaminants.
Cold welding may also be used to seal a metal enclosure, or special adhesives can be used to seal metal or other materials which form the enclosure. However, these structures also require considerable physical space for the sealed product and can be a costly solution, depending on the complexity of the assembled components and the degree of hermeticity required.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a cover for a circuit board that cooperates with the circuit board to provide a sealed circuit board assembly. It is also desirable to have such a cover and sealed circuit board assembly wherein electrical components on the circuit board are sealed from the ambient environment while still providing access to connector assemblies mounted on the circuit board. It is also desirable to have such a cover and sealed circuit board assembly that does not require increased space within an instrument housing in which the circuit board is mounted.